Initial skeleton of high-contrast mode.

third_party/WebKit/Source/core/frame/Settings.json5

 {
   // Defines properties which are available on the Settings object.
   //
   // Please think carefully before adding a new Setting.  Some questions to
   // consider are:
   // - Should this be a RuntimeEnabledFeature instead? Settings are for things
   //   which we support either values of at runtime.  Features are set at renderer
   //   process startup and are never changed. Features also tend to be set to a
   //   value based on the platform or the stability of the code in question, where
   //   as settings both codepaths need to be stable.
@@ skipping 909 matching lines @@
       initial: true,
       invalidate: "MediaControls",
     },
 
     // Whether we should not update selection attributes when mutating selection range.
     // TODO(changwan): remove this flag when we no longer support Android M.
     {
       name: "doNotUpdateSelectionOnMutatingSelectionRange",
       initial: false,
     },
+
+    //
+    // High contrast mode
+    //
+    {
+      name: "highContrastMode",
+      initial: "HighContrastMode::kOff",
+      type: "HighContrastMode",
+    },
+    {
+      name: "highContrastGrayscale",
+      initial: false,
+    },
+    {
+      name: "highContrastContrast",
+      initial: 0,
+      type: "double",
+    },
   ],
 }